It is often necessary to ascertain the composition of various substances. For example, there are many important applications for real-time, on-site measurements of compounds in various environments. These include measurements at toxic waste sites, work places, industrial sites, accidental spill sites, and semiconductor fabrication facilities. A gas chromatograph, either alone or in combination with a mass spectrometer, can be used to take such measurments.
A Gas Chromatograph ("GC") separates a sample mixture into different components according to some specified parameters. The process separates the different components spatially, causing each material to arrive at the output of the Gas Chromatograph at a different time.
These separated materials are fed to a Mass Spectrometer ("MS"). The MS allows spatially-separated materials to be individually processed by the mass spectrometer; i.e., one material can be processed at a time. The MS analyzes each single material to determine its mass spectrum. The mass spectrum of a compound consists of the intensities of different mass-ions originating from the parent molecules and their fragments. The spectrum is characteristic of the chemical compound and is used for its identification and quantitative measurement.
GCMS systems have historically been extremely large and unwieldy devices. They need high power for operation and have been extremely high in cost.
A mass spectrometer operates by ionizing a gaseous/vapor sample of material. FIG. 1 shows sample vapor being introduced into the ionization source 112 either directly, or more preferably, through a gas chromatograph 110. The gas chromatograph is preferably used for a complex mixture.
The ion source is maintained under vacuum at a pressure of .about.10.sup.-5 torr with a vacuum pump. The sample molecules are bombarded with a beam of electrons in the ionization source. The process results in the production of ions of various masses depending on the chemical nature of the sample molecules. The ions are then separated according to their masses (charge to mass ratios) by the application of electric and/or magnetic fields. Intensities of different mass ions are measured by using a detector system 116.
The gas chromatograph portion of a GC mass spectrometer has typically used a coated capillary tube. The tube is coated with polymeric materials. An inert carrying gas is passed through the capillary tube. The elements of interest--collectively called the analyte--is passed into the inert carrying gas. Each of the components of interest within the analyte have different affinities with the coating on the capillary tube. This affinity changes the flow velocities of the passage of those components down the capillary column.
Normally the operation progresses as follows. The inert gas is continuously flowing through the capillary tube. A measurement cycle is initiated by adding a "slug" of analyte. The analyte includes components with different affinities with the coating. Those different affinities change the velocity of the different components of the analyte. The different components hence arrive at the output of the gas chromatograph at different moments. Each element arriving at the output is analyzed by the mass spectrometer.
The gas chromatograph tubing has typically been a 250-500 micron diameter tubing with 2-5 atm.multidot.cm.sup.3 /s of gas flowing therethrough. This volume of gas through the gas chromatograph enters into the mass spectrometer and necessitates a large vacuum pump with high pumping speed to maintain the proper low pressure within the mass spectrometer. An object of the present invention is to minimize the amount of gas which flows therethrough.
The inventor recognized that amount of gas which flows through the column can be reduced by narrowing its diameter. However, the art has generally suggested that narrowing the pipe is undesirable. One reason why those having ordinary skill in the art previously have not narrowed the diameter is because of the problems associated with narrowed GC effluent peaks. When the diameter of the column of a gas chromatograph is narrowed, the peak-widths of analytes emerging from the column are also narrow. Scanning-type mass spectrometers, which typically lose a large percentage of the signal, have been unable to make multiple mass spectral measurements of these narrow peaks. It is an object of the present invention to obviate these problems using special new techniques described according to the present invention.
Mass spectrometers can be of a scanning-type or of a nonscanning-type (focal plane type). A scanning-type MS separates the different mass ions in time. Each intensity is measured successively by a single element detector. The ions of all the other masses are discarded during the time while the intensity of one mass is measured. A focal plane type MS, in contrast, spatially separates ions of the different masses. The intensities of these spatially-separated ions are measured simultaneously with a photographic plate, or an array detector, having multiple elements, of high sensitivity and spatial resolution.
A block diagram of the scanning type mass spectrometer is shown in FIG. 2. The quadruple mass spectrometer shown in the figure is a typical example of this type of MS. Ions are produced from an ion source 200 and the output ions enter a tuned cavity 202. Cavity 202 is tuned to allow only a single mass ion 204 to pass; all the other untuned ion masses 206 are discarded in order to resolve only the tuned mass ions. The tuning of the cavity is scanned over time. This means that different ion masses are successively allowed to pass at different times. At any given time, therefore, only a single ion mass will hit the detector 210 e.g., an electron multiplier. The intensity of the ions measured by the detector, therefore, indicates the amount of ions of that mass in the sample.
Scanning over the whole mass spectrum enables determination of a plot of mass versus intensity. Each particular material is formed from a unique combination of different masses and their intensities. The combination is called a mass spectrum 118. Thus, the scanning plot (mass spectrum) provides the chemical nature of the material.
Scanning-type devices de-tune most of the ions at any given time. Hence, most of the signal generated from a sample is deliberately lost prior to detection. These devices have limited scan rate and possess relatively low sensitivity.
The focal plane type of mass spectrometer spectrally analyzes all the different mass-ions from the sample at once. The mass spectrometers based on Mattauch-Herzog ("M-H") geometry or Dempster geometry are examples of this type of MS.
FIG. 3 schematically shows an array type MS of the Dempster design. A magnetic field in the magnetic analyzer 303 is used to separate the different mass ions. Each ion mass is directed to a different location 304, 306 along the focal plane. An array of detectors with high spatial resolution is placed along the focal plane to measure the intensities of all the ions simultaneously. Signals from different detector elements provide the intensities of different mass ions. The individual detector elements of the array detector for this focal plane geometry need to be small so that signal measurements with spatial resolutions of 10-30 microns can be accomplished. Multiple detector elements cover the region of each mass ions. The intensity/peak profile of each mass is thus obtained from the detector output.
Both types of mass spectrometers measure a characteristic spectrum of intensity versus mass. As described above, this spectrum can be used to identify the compound.
GCMS arrays have broad uses. However, the high cost of using a GCMS system has often prevented the GCMS from being used in certain operations. This high cost is not only based on the hardware; GCMS systems are very heavy and hence difficult to transport. Reducing the size and hence weight of the device can therefore significantly reduce the cost of transportation.